planning to buy GPS

[jon.aurther]

Hi, Hi, I am planning to buy GPS, I know there are different generations of GPS antenna. I heard about SIRF ?? 1 2 and 3 etc etc. and I am still not totally clear about them.. I read a lot customer reviews on amazons and stuff.. but they r all not clear. I want to buy a gps but I don't want to be tricked to buy GPS with old tech- and keep losing signal and stuff. Most gps- r sold without info / description of their antenna technology!!!

Anyone is expert in these..??

[thuor]

GPS Antennas

That's actually a good point - technically referred to as "urban canyon" phonomenon - which is whereby the receiver sensitivity is insufficient in areas with large building obstructions as well as other geophysical features as well as indoors and as other RF shielded locations. In this situations the antennas is important as is the specific radio architecture - and by radio we mean the RF front end that detects and demodulates the satellite signals (at 1575.42 MHz). Receiver sensitivity in RF circuit design is roughly the minimum signal the system can receive at a predetermined (and low) error rate.
So to understand your situation its important to understand your application and usage scenario. Unfortunately low end hand-held gps units do not have external antenna connectors and in any case other gps parameters such as the number of channels, screen resolution, application support etc are more important than receiver sensitivity. Many in car gps units have external antenna connectors and allow mounting of gps antennas exterior to the vehicle for maximum sky exposure. In this case you may add a higher gain antenna as long as the connector type matches the system connector type. In many cases the gps antennas that come standard with these units are more than sufficient and we recommend people to try the existing antenna before going out and buying a higher gain external antenna. A lot of times the signal is either too weak or the receiver sensitivity too poor to improve it with external antennas.
Another common approach that has recently become popular to circumvent the urban canyon phonemenon is A-GPS (Assisted GPS). This is where an external reference source such as an assistance server on a WiFi or Cellular network helps out the gps unit which is important when the gps unit is having satellite acquisition issues.